The present invention relates to a probe head for a coordinate-measuring instrument wherein deflectable probe-mounting structure defines a three-dimensional coordinate system so that a work-contacting probe pin can be deflectable in all of three coordinate directions in space.
Coordinate-measuring instruments serve to detect with high precision the space coordinates of a workpiece at preselected locations on the workpiece, and to display and/or feed measured data to an associated computer. The computer determines the desired measurement of the workpiece from the measured coordinate values.
To carry out this measurement task, a probe head displaceably mounts a probe having a work-contact pin, and the probe head is mounted to a coordinate-measuring instrument for displacement in all three directions in space. The space coordinates of the probe pin at any moment are known from measurement-value transmitters which are connected to and form part of the measuring instrument. The means for thus displacing the probe head can be actuated manually but are usually motor-driven, to effect controlled three-dimensional displacement of the probe head. Such a coordinate-measuring instrument is known, for example, from West German Patent 2,718,506 and its corresponding U.S. Pat. No. 4,175,327.
The probe head itself has the task of signalling retention of the instantaneous space coordinates of the tip of the probe pin as soon as said tip contacts the workpiece to be measured. Thee coordinate values are then fed to an evaluation unit which computes the desired dimensions of the workpiece from the coordinate values determined at different points of workpiece contact.
The probe head itself must be so developed that probe-pin contact with the workpiece (and the coordinate measurement concomitant with such contact) are possible from all three directions in space. In the contacting process, the mechanism for moving the probe head cannot be brought to a stop at the precise instant of probe-tip contact with the workpiece; it is therefore necessary to so mount the probe in the probe head as to permit probe-pin deflection in all directions of workpiece contact, i.e., in all three space coordinates.
A large number of probe heads are known which are distinguished by the manner in which a determination is made of space-coordinate values associated with the contacting process. In general, one distinguishes between two categories, namely switching probe heads and measuring probe heads.
The first category, namely, the so-called switching probe heads, includes those which upon the first contact of the probe tip with the object being measured initiates a trigger pulse which retains the instantaneously operative coordinate values.
Such a probe head is illustratively known from West German Patent 2,712,181 and its corresponding U.S. Pat. No. 4,177,568. The probe head has a support which determines the zero position of the probe pin with high precision and to which the probe pin is allowed to return, by a corrective repositioning of the probe head after each deflection. To generate the trigger pulse at the instant of workpiece contact, a highly sensitive sensor, such as a piezoelectric element, is connected to the probe pin.
In the work-contacting procedure, the probe head is advanced by the coordinate-measuring instrument toward the workpiece until the tip of the probe contacts the workpiece, whereupon the sensor initiates the trigger pulse. The trigger pulse transiently freezes the coordinate values which existed at the instant of workpiece contact. As the probe head momentarily continues its advancing movement, the probe pin is deflected from its zero position. This deflection, via electro-optical means, initiates a second pulse which verifies the trigger pulse, i.e., enters the transiently frozen coordinate values in the computer, and at the same time initiates the process of braking the probe-head drive of the coordinate-measuring instrument. In this connection, the probe pin comes to a deflected standstill, and the probe-head drive of the measuring instrument is reversed, to reposition the probe head until the probe pin has resumed its zero position. The measuring instrument is then ready for the next work-contacting procedure, at a different location on the workpiece.
A switching probe head of the indicated character is relatively inexpensive and is to a large extent instrument-compatible, since detection of the measurement value occurs at the instant of probe-pin contact with the object being measured.
The second category includes the so-called measuring probe heads. They illustratively comprise a probe-mounting system which consists of a torsionally rigid sequence of play-free and friction-free linear suspensions, wherein each suspension accommodates deflection in a different one of the coordinate directions of a three-dimensional coordinate system. These linear suspensions are advantageously developed as spring parallelograms, incorporating installed direction-sensitive and position-sensitive signal transmitters which supply signals upon deflection of the probe pin out of its zero position. These signal transmitters are so integrated with a position-control circuit that, commencing with the instant of initial probe-tip contact with the object being measured, (1) probe-head repositioning data accumulate for probe deflection after initial workpiece contact, and (2), upon attaining a predetermined probe deflection (or a predetermined time after initial workpiece contact), the measurement is deemed to have been validated and is automatically entered into the computer of the coordinate-measuring instrument.
Such a measuring probe head is known, for example, from West German Patent 2,242,355 and its corresponding U.S. Pat. No. 3,869,799.
The position-control circuit for this known probe head can be developed in such manner that, after each contact with the workpiece, the probe head is returned in controlled fashion either to its zero position or to a starting position in which a preset work-contacting measurement force is reached, it being noted that any mechanical oscillation occasioned by work contact will have dissipated by the time of return to the zero or to the starting position.
It is also possible to so develop the position-control circuit that return to zero position is dispensed with, and so that the computer combines the position signals with the coordinate signals of the coordinate-measuring instrument. This permits continuous contacting of an object to be measured along a predetermined line, i.e., so-called "scanning". In this connection, the actual coordinates of the object being measured are determined by computer at each point along the scanning path.
Measuring probe heads of the character indicated are costly in view of the required high mechanical and electronic expense. Generally, they are also not instrument-compatible and are not easy to handle.
Each of the two indicated probe-head systems, namely, of the switching type and of the measuring type, has characteristic advantages and disadvantages, calling for application to different industrial measurement tasks. The desire has therefore arisen for a probe system which permits both the measurement mode of operation and the switching mode of operation. A first solution for such a probe system is described in West German Patent No. 2,835,615, wherein a probe head includes a light source which provides at the probe pin a beam which forms an inward extension of the probe-pin axis. Within the probe head, this light beam strikes a position-sensitive photodiode so that probing deflection can be measured as with a measuring probe head, by means of analog signals. But such a measurement is possible only in one plane, i.e., in two coordinate directions, and furthermore cannot be carried out with the precision obtainable in modern coordinate-measuring instruments. In the Journal, "Messen und Prufen/Automatik", January/February, 1984, Pages 40/45, it is stated that the probe known from West German Patent No. 2,835,615 can also be used as a switching probe; to this end, at Section 3.3.2 (Page 43), it is explained that a coincidence circle is centered on the point of origin of the two-dimensional coordinate system of the probe, and the radius of this coincidence circle defines a constant value of probe-pin deflection. A microcomputer continuously, at brief intervals, tests whether the probe axis is passing through the periphery of this circle. If so, then the contact coordinates are accepted for entry into the computer.
This known probe is suitable only for the two-dimensional contacting of an object to be measured and requires a relatively high electronic expense.